It is very often advantageous in a digital communication receiver to use feedback between constituent functional blocks. This is especially true in the case of wireless receivers where the cooperative operation of the channel estimation/equalization block with the subsequent decoding block can lead to significant performance improvements. Such schemes bear different names such as turbo equalization, successive interference cancellation, or the like. This cooperative operation is often performed in an iterative fashion.
In order to have this kind of feedback operation feasible, soft bit information, usually in the form of log-likelihood-ratio (LLR) corresponding to all the bits at the turbo decoder output have to be available. This includes both systematic bits (i.e., information bits) and parity bits. However, in a conventional implementation, a turbo decoder iteratively computes LLR values only for the systematic (i.e., information) bits. These are then hard decoded and sent out for further processing.
Typical turbo decoders do not provide the information that would be required for the implementations of advanced receiver techniques (e.g., turbo equalization, advanced interference cancellation techniques, etc.) in digital communication receivers, especially in the area of wireless communications. In particular, a conventional turbo decoder does not compute any kind of soft output related to the parity bits of an encoded data stream. In other words, a typical turbo decoder does not have the capability to provide LLR (soft output) values for parity bits, which is the form that a receiver system would require soft output information.
It would therefore be desirable to provide a turbo decoder circuit that provides soft outputs (i.e., LLR values) as feedback for both systematic bits and parity bits. It is further desirable that this circuit do so using only input LLR values received by the turbo decoder, and a fed back systematic soft output from the turbo decoder.